The 280km/h "M-Driver"option doesnīt meen delimited - they only increase the limit, because the M5 could reach 305km/h with this option and they wonīt hurt the bigger car (and sales).
According to my dealer fully delimited 310km/h are posible. Also he claims that the M-driver option wouldnīt be installed as soon as the car is more than 2000km old.

As much as I don't like the look, I'll probably end up keeping it... I'd guess the M3 front clip generates additional downforce, so I'm guessing the rear lip spoiler is there to keep things equal.

It's to minimize the Caonda effect. It reduces drag and maybe slightly uplift. It's too small to produce downforce.

But, it should have a slight effect on mileage at speed and top speed. Henri Coanda was an Italian aerodynamacist that discovered this. See pages 137 - 138 of the 7/01 Road & Track. There's a diagram of a spoiler exactly like the M3s showing the effect.

It's to minimize the Caonda effect. It reduces drag and maybe slightly uplift. It's too small to produce downforce.

But, it should have a slight effect on mileage at speed and top speed. Henri Coanda was an Italian aerodynamacist that discovered this. See pages 137 - 138 of the 7/01 Road & Track. There's a diagram of a spoiler exactly like the M3s showing the effect.

Yep, there is no way that little plastic spoiler can create significant downforce; little surface area and not a structural component. It's mainly there to manage the flow. Turbulance/undesired pressure differences are not good things in general unless you are young and stupid like I was 15 years ago and got a kick out of riding a bicycle behind trucks at 120kph on highways.